Vehicle Exhaust Emissions Benefit from a Regulatory Cap in Gasoline Distillation Index

The Distillation Index (DI) is a measure of the volatility of gasoline, especially its tendency to vaporize in an engine at initial start-up and during warm up. On January 27, 1999 the U.S. domestic and import automotive manufacturers petitioned the US EPA to limit the DI of all U.S. gasoline to 1200 degrees Fahrenheit as a means of reducing in-use emissions and ensuring consistent cold start and warm-up driveability.[1] Air Improvement Resource, Inc. (AIR) completed a 1999 study that evaluated the benefits of a DI cap. Overall, the 1999 AIR study estimated that the DI cap would produce a 16 and 15 percent reduction in hydrocarbon (HC) and carbon monoxide (CO) exhaust, respectively, from gasoline vehicles nationally in 2020. [2]

In 2000, the Alliance of Automobile Manufacturers sponsored a more compreshensive examination of the emission consequences of the DI cap on which this paper is based. In this paper, the results include an evaluation of 1999 gasoline survey data and an accounting for potential future changes in oxygenate consumption - for example the elimination of MTBE use in gasoline. This updated analysis estimates that the DI cap would produce a national benefit of 20 to 23 percent for exhaust HC and 25 percent for exhaust CO. Thus, the need for a DI cap and the benefit from such a cap have increased over that estimated in 1999. Moreover, this paper also shows that the magnitude of the DI cap benefit varies regionally according to gasoline type sold. Areas subject to Federal reformulated gasoline requirements potentially would realize the greatest emission inventory benefit from the proposed DI cap - up to a 28 percent reduction in HC exhaust from gasoline vehicles in 2020, and these are areas where the benefit would be the most beneficial to ambient air quality.